Binding of cerebrosides and sulfatides to saposins A-D

Saposins are a family of four small glycoproteins, all of which are derived from prosaposin, and are involved in the lysosomal hydrolysis of various sphingolipids. Results from this investigation demonstrate that saposins A-D bind to galactosyl- and glucosylceramide. The binding was highly dependent on the solution pH; maximum binding of glucosylceramide to all saposins occurred at pH 7. Maximum binding of galactosylceramide to saposins B and D occurred at a more basic pH (8.5). The binding of glucosylceramide to saposins was significantly inhibited by Mg2+, Ca2+, or Zn2+. Although maximum binding of sulfatide to saposins A, C, and D occurred at acidic pH, the binding to saposin B was maximum at pH 8.5. Saposin A also bound sphingomyelin or phosphatidylcholine at neutral pH. No significant binding was evident between these lipids and saposins B-D at any pH value. The existence of saposin-lipid complexes was further confirmed in selected samples by gel filtration, isoelectric focusing, and a TLC binding assay. We have also shown that galactosylceramide bound to saposins A-D was efficiently transported to a rat brain microsomal fraction. This result suggests that saposins and possibly their precursor, prosaposin, may be involved in membrane biogenesis such as the assembly of myelin and plasma membranes.